Our global hypothesis is that "functional" tissue-engineered constructs containing mesenchymal stem cells can be used in models of tendon injury to improve repair biomechanics, structure, and biochemistry. Implants will be designed in vitro with appropriate contraction characteristics and initial biomechanics to improve tendon repair. Functional tissue engineering or FTE will be used to mechanically stimulate the tissue engineered (TE) implants in culture and to establish thresholds of in vivo forces that the implants will be expected to transmit after surgery. We will address four research questions. First, can we significantly improve repair outcome by varying the in vitro culture conditions? Second, what are the thresholds of in vivo force transmitted by normal tendons and how are these forces distributed through the repair site after implant surgery? Third, would altering the mechanical signals applied to the constructs in vitro enhance repair biomechanics after surgery? Finally, do in vitro markers exist that correlate with in vivo outcome, both short and long term? In a series of in vitro and in vivo studies, the following Specific Aims will be examined: 1. Create MSC constructs for up to 6 weeks in culture using selected combinations of cell-seeding density, collagen concentration, and collagen braids as suture replacements. Evaluate in vitro contraction characteristics, initial biomechanical properties, cell morphology, collagen synthesis, and expression of alkaline phosphatase and osteocalcin. 2. Establish in vivo loading regimens on normal Achilles and patellar tendons for various activities. 3. Estimate load distributions in the repair tissues for different combinations of implant conditions using finite element modeling. 4. Evaluate in vivo selected combinations from SA1 in light of design criteria derived from SA2 and SA3 to determine the success of the implant in replacing the injured tissue. 5. Assess the potential benefits of continuous and intermittent cyclic strain on construct contraction, initial biomechanical properties, cell morphology, and biochemistry in vitro. 6. Evaluate in vivo the most promising combinations from SA5 in light of the previous aims to determine the potential benefits of in vitro mechanical stimulation on repair biomechanics, histology, and biochemistry after surgery. 7. Identify possible predictors of in vivo outcome by correlating in vitro biomechanical, histological, and biochemical results with early and long-term in vivo outcomes.